Suspended carbon nanotube beams fabricated by a low temperature surface micromachined process for humidity and gas sensing

Abstract

The world we live in today is connected in more ways than we can imagine. Smart devices and networks form the core of our everyday lives. The advent of the Internet of Things and Industry 4.0, which are terms defined to make the world around us more connected via the internet at a domestic and industrial setting respectively, necessitates various kinds of sensors working in conjunction with each other. Keeping this in mind, this thesis explores the application of Suspended Single walled Carbon Nanotubes for Gas Sensing. Gas Sensors are used in a variety of environments such as hospitals, space shuttles and domestic kitchens to detect and prevent the leakage of hazardous gases. A variety of materials such as Polymers, ceramics and metal oxides and detection mechanisms have been used to identify various gases. More recently, Carbon nanotubes have garnered considerable attention for gas sensing applications due to their excellent electrical properties and ease of surface modification. With advancements in production techniques, the cost of Carbon Nanotube production has reduced significantly which has enabled the development of Carbon Nanotube based gas sensors. Although multiple Carbon Nanotube based gas sensors have been studied in the past, the overall device structure remains broadly similar. Additionally, these devices offer suffer from large hysteretic effects, often because of the effect of the substrate on the overall sensing mechanism, which can lead to unreliable measurement outputs. This work aims to solve the issue of hysteresis by using suspended Carbon Nanotubes and thereby isolating the sensing element from the effects of the substrate. To do this, first, a low temperature surface micromachining process is developed to obtain suspended devices. Once the process is well developed, the suspended device is used to detect humidity. The performance of the suspended device is compared to a non-suspended device to demonstrate the advantages of suspended Carbon nanotubes. Next, the performance metrics of the suspended sensor are improved by chemically functionalizing the nanotubes to make them more sensitive to water vapor. The nanotubes are obtained commercially, pre-functionalized, and used in the fabrication process. A comparative study between suspended and non-suspended functionalized nanotube-based sensors is also done. Lastly, the device design has been modified to fabricate an ionization gas sensor based on horizontally aligned suspended carbon nanotubes. The ionization potential of various gases is identified and other parameters such as the repeatability, long term stability, etc. have been studied.

Date	29 Mar 2020
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Nabki (Supervisor) & Ricardo Izquierdo (Co-supervisor)